This invention relates to an electronic thermometer and more particularly to a noncontacting infrared electronic thermometer and method for measuring temperature of an object without contact with it.
The temperature of an object, such as the human body, can be determined by using a contact thermometer or by measuring the natural thermal radiation from it surface in infrared and far infrared spectral ranges. The radiation is related to temperature of the object and can be utilized to determine the temperature of the body.
Numerous devices have been proposed for making noncontact temperature determination. Among them the U.S. patents issued to Wormser U.S. Pat. No. 2,798,962, Derganc U.S. Pat. No. 2,920,485, Siegert et al U.S. Pat. No. 3,023,398, Barnes U.S. Pat. No. 3,282,106, Hewett et al U.S. Pat. No. 3,303,430, Treharne U.S. Pat. No. 3,344,739, Paddock U.S. Pat. No. 3,463,006, Wortz U.S. Pat. No. 3,581,570, Risgin et al. U.S. Pat. No. 3,777,568, Michael U.S. Pat. No. 4,005,605, Berman et al. U.S. Pat. No. 4,372,690, Crane et al. U.S. Pat. No. 4,470,710, Irani U.S. Pat. No. 4,525,896, O'Hara et al. U.S. Pat. No. 4,602,642.
The prior art infrared thermometers use a well known concept, as exemplified by the teachings of the above mentioned U.S. Pat. Nos. 2,798,962; 2,927,464; 3,920,485 and 4,470,710. The instruments include one or more "reference" radiation sources of constant and known temperature, a detector which is capable of measuring radiation from both target and "reference" sources and a "chopper" to alternate radiation sources. A different solution is the use of a moving reflector (U.S. Pat. No. 4,005,605). Another principle, which was used to produce the infrared thermometers, is based on monitoring of ambient temperature to compensate for changes in detected radiation flux (U.S. Pat. No. 4,527,896). U.S. Pat. No. 3,581,570 issued to Wortz describes a radiation thermometer where a thermistor or other thermal sensor is placed into the focal point of a parabolic mirror in order to collect radiation flux from an ear canal. Barnes (U.S. Pat. No. 3,282,106) claimed germanium or silicon lenses to focus infrared radiation on the radiometer, U.S. Pat. No. 4,602,642 issued to O'Hara et al. teaches warming up of the thermopile sensor by an external heating source to the precise temperature of an object and by using an additional thermistor to regulate heating of thermopile. Another patent of O'Hara et al. U.S. Pat. No. 4,662,360 discloses a disposable speculum which can be placed over the thermometer probe to use it as a sanitary barier in medical applications.
A number of the prior art disclosures (for instance, U.S. Pat. No. 3,581,570) cited above mention thermistors as means to detect infrared radiation. None of them succeeded to overcome a major problem of that sensor, which is slow speed response and, to the applicant's knowledge, none of the thermistor based infrared thermometers have been successfully reduced to practice. All infrared thermometers known in the prior art have the following limitations: dependence on the linearity of the radiation sensor, strong sensitivity to ambient temperature which required use of multiple sensors and moving parts or necessity of an additional temperature sensing means and/or heating elements and thermostats. Most of the radiation thermometers, which were reduced to practice, are large and cumbersome instruments.
It is an object of the present invention to provide a new and improved noncontacting electronic thermometer which is accurate, reliable, small in size, has no moving parts and economical to manufacture.
Another object of the invention is to provide a noncontacting electronic thermometer for medical use which is compact, fast, inexpensive and convenient to use.
A further object of the invention is to provide a method for measuring temperature of an object by utilizing a self-heating temperature sensitive resistor with electronically controlled resistance.